I think it's up to the Supreme Court to decide what the remedy is, were it to make such a decision. Some court decisions on citizenship are retroactive, and some are not, but I think there is reason to believe in this hypothetical case, they would not apply it retroactively.
In the case of cases regarding loss of citizenship, I believe that decisions have been retroactive. In Afroyim v. Rusk (1967), the Supreme Court decided that it was unconstitutional under the 14th Amendment for someone born or naturalized in the US to lose US citizenship unless they intended to relinquish US citizenship (or unless they obtained naturalization unlawfully, which arguably is not loss of citizenship since they never validly had it). 7 FAM 1240 Appendix B (c) says that the decision was "retroactive in effect", i.e. anyone who was told they lost citizenship in the years before the decision (presumably all the way back to the enactment of the 14th Amendment in 1868) according to the statute at that time, would be considered to have not lost it if they did not intend to relinquish it. Here the retroactive application of the decision preserves people's citizenship -- it nullifies the loss of citizenship.
However, there is a recent Supreme Court case on citizenship at birth, where the Supreme Court did not apply the decision retroactively. In Sessions v. Morales-Santana (2017), the Supreme Court ruled that the provision which specified a different physical presence requirement for children born abroad out of wedlock to American mothers (1 continuous year of physical presence in the US), as opposed to the physical presence requirement for children born abroad in wedlock to 1 US citizen parent and 1 alien parent, or for children born abroad out of wedlock to American fathers (5 cumulative years of physical presence, including 2 years after turning 14) was unconstitutional. As a remedy, the Supreme Court decided that, "prospectively", the 5 cumulative years standard would be applied to children born out of wedlock to American mothers also. But it did not retroactively apply the 5 cumulative years standard to children born out of wedlock to American mothers before the date of the decision -- in other words, the Supreme Court decided to continue applying a section of law that it decided to be unconstitutional, to children born before the date of the decision.
The Supreme Court decision did not say why the remedy was not retroactive in effect. In my opinion, it is probably because it is generally considered unfair and harmful to retroactively take away citizenship from people, which they thought they had based on the best interpretation of the rules at the time; on the other hand, retroactively granting or restoring citizenship is considered much more acceptable, as it is extending a benefit retroactively. In the case of the children born out of wedlock to American mothers, retroactively changing it to the 5 cumulative years condition would take away far more people's citizenship than it would grant (it is much more likely to meet the 1 continuous year requirement than to meet the 5 cumulative years requirement). This would also be consistent with how, historically, statutory changes to acquisition of citizenship for children born abroad have generally not been retroactive (see 8 FAM 301.4-1(A)(2)); and in the rare cases there have explicitly been retroactive changes, it has always been to grant more citizenship, not take it away.
Although a hypothetical decision where birthright citizenship was limited wouldn't completely parallel the Sessions v. Morales-Santana decision (in Sessions v. Morales-Santana, there was no dispute about the interpretation of the statute; rather, the statute was determined to be unconstitutional; in a hypothetical decision where birthright citizenship was limited, the statute would not be unconstitutional; rather, they would have to decide that both the statute and constitution, which both grant birthright citizenship, have been interpreted incorrectly), the reasons I have mentioned above point to it not being applied retroactively. Retroactively applying such a limitation would take away citizenship to lots of people, who believed they had it based on what almost everyone believed was the correct rule at the time.